As an effective method for synthesizing a material of nanometer size such as a carbon nanotube or a semiconductor nanowire in large quantity, there is known a chemical vapor deposition method (CVD method) in which a carbon-containing raw material containing a catalyst with a very small particle diameter is atomized, and introduced into a high-temperature electric furnace by a spray or the like for synthesis.
When metal ultrafine particles in nanometer size are used among the catalysts for use in a conventional CVD method, it is known, in addition to the advantage of the high catalytic activity caused by the large surface area of the catalyst, that the diameter of the resulting carbon nanotube can be controlled to a certain degree by the diameter of the metal ultrafine particle in the CVD synthesis of carbon nanotubes (see, Non-Patent Document 1).
However, the metal ultrafine particle catalyst in nanometer size, especially, the ultrafine particle catalyst of a metal having magnetic properties tends to aggregate due to the electric, and magnetic interactions and the like acting between the particles. Thus, there has been a problem that the catalytic performances tend to be degraded.
In order to prevent aggregation of the catalyst ultrafine particles, there have been proposed: a method in which metal ultrafine particles are simply dispersed in an organic solvent by using a surfactant (see, Patent Document 1); and a method in which metal ultrafine particles are prepared in a microemulsion, thereby to be dispersed in an organic solvent (see, Patent Document 2).
However, even with the CVD method using these catalysts, when a raw material solution containing a catalyst is introduced into a narrow channel, the dispersibility of the catalyst particles is remarkably reduced, so that the catalyst particles aggregate with each other to clog a spray nozzle. Not only this problem, but also the following serious problem is caused. Aggregation remarkably reduces the catalyst surface area, which degrades the catalytic performances. This causes the reduction of the yield.
Non-Patent Document 1: Journal of Physical Chemistry B, vol. 106, 2002 (issued on Feb. 16, 2002), p2429
Patent Document 1: JP-A-2-6617
Patent Document 2: Japanese Patent No. 3438041